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At 11:50 am on October 15, 1970, when steelworkers on Melbourne, Australia’s Westgate Bridge removed 30 supporting bolts in order to help line up two bridge sections, the 112-meter span fell, killing 35 people. The extent of the problems that caused the accident, however, ran much deeper than this one final action.

Mismanagement, Miscommunication, Bad Engineering

The Westgate Bridge spans the Yarra River in Melbourne, Australia, with 4 lanes of vehicle traffic in each direction. It runs a total of 2582 meters, with a central, steel section over the river of 849 meters rising 58 meters above the river, 37 meters wide. It is designed as a cable-stayed box girder bridge, with two central towers standing 46 meters above the bridge deck running cables to the center of the bridge. Stiffened and reinforced box sections, bolted together transversely and down the center, make up the body of the bridge. It is designed to handle 40,000 vehicles per day, up to 25 tons per vehicle.

Too many parties were involved in the bridge’s construction. The Lower Yarra Crossing Authority was the overseeing body. Melbourne’s Maunsell & Partners and London’s Freeman, Fox & Partners were the engineering firms, John Holland Construction (Melbourne) and World Services and Construction (Netherlands) were the main contractors. Because of labour unrest and strikes, communication difficulty between the companies, and construction delays, contracts and obligations for fabrication and erection of steelwork and concrete sections shifted between companies. The question of who did what, under whose guidance, eventually became complex and cumbersome.

Box girder bridges have proven notoriously difficult to build correctly. Slender steel plates are used to conserve weight, with the hollow box design below used to provide strength. Exacting tolerances and engineering are required at every stage to ensure the final product is solid and useable. Between 1969 and 1971, 4 box girder bridges failed worldwide, including a collapse in Wales with a design similar to Westgate. These lessons failed to alert the Westgate engineers and workers to the danger of their methods.

The central spans were difficult to put in place. The established plan was to build two sections of a span in full length and half width on the ground, then crane them into place and bolt them together. This technique required ongoing, precision monitoring to assure a uniform fit. This monitoring was not done. When the sections between Pier 10 and Pier 11 were lifted, they were found to be 11.4 cm apart from each other. On lifting, one of the sections experienced a minor buckling, a defect which workers figured they could reinforce once the section was bolted in.

After discussions, on-site contractors decided to weight the higher section with ten 8-ton concrete blocks, a dubious engineering technique known as kentledge. This resulted in a major buckling, and work stopped for discussion. It was decided to remove bolts on both sides of the buckle to allow it to settle. Some of the bolts were under such strain that they had to be broken off to remove them, but the significance of this was not realized. At 11:50 am on October 15, 1970, when 30 bolts had been removed, the span snapped, sending the 112 meter, 2000-ton span 50 meters to the ground and water. 35 workers were killed, some on the span and some in huts under the bridge.

The Aftermath

Buildings shook for hundreds of meters, and the resulting fire took the rest of the day to extinguish. Damage to the bridge and area amounted to $10 million. The Australian government established a Royal Commission to investigate the accident. In its 300 page report the Commission laid blame on practically everyone involved with the project. In part, it stated, Error begat error … and the events which led to the disaster moved with the inevitability of a Greek Tragedy.’ - (Report of Royal Commission, VPRS 2591/P0, unit 14). The bridge was redesigned to exacting standards, work resumed in 1972, and the bridge was completed in 1978. Total cost was $202 million.

Links

http://www.youtube.com/watch?v=UR8eYevYcg8

http://www.prov.vic.gov.au/exhibs/westgate/welcome.htm

http://www.westgatebridge.org/WestGateBridge_Study.pdf

 

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